Gas separation is important in various industries and can typically be accomplished by flowing a mixture of gases over an adsorbent that preferentially adsorbs a more readily adsorbed component relative to a less readily adsorbed component of the mixture. One of the more important gas separation techniques is pressure swing adsorption (PSA). PSA processes rely on the fact that under pressure gases tend to be adsorbed within the pore structure of a microporous adsorbent material or within the free volume of a polymeric material. The higher the pressure, the more gas is adsorbed. When the pressure is reduced, the gas is released, or desorbed. PSA processes can be used to separate gases in a mixture because different gases tend to fill the micropore or free volume of the adsorbent to different extents. If a gas mixture, such as natural gas, for example, is passed under pressure through a vessel containing polymeric or microporous adsorbent that fills with more nitrogen than it does methane, part or all of the nitrogen will stay in the adsorbent bed, and the gas coming out of the vessel will be enriched in methane. When the adsorbent bed reaches the end of its capacity to adsorb nitrogen, it can be regenerated by reducing the pressure, thereby releasing the adsorbed nitrogen. It is then ready for another cycle.
Another important gas separation technique is temperature swing adsorption (TSA). TSA processes also rely on the fact that under pressure gases tend to be adsorbed within the pore structure of a microporous adsorbent material or within the free volume of a polymeric material. When the temperature of the adsorbent is increased, the gas is released, or desorbed. By cyclically swinging the temperature of adsorbent beds, TSA processes can be used to separate gases in a mixture when used with an adsorbent that selectively picks up one or more of the components in the gas mixture.
Adsorbents for PSA systems are usually very porous materials chosen because of their large surface area. Typical adsorbents are activated carbons, silica gels, aluminas and zeolites. In some cases a polymeric material can be used as the adsorbent material. Though the gas adsorbed on the interior surfaces of microporous materials may consist of a layer only one, or at most a few molecules thick, surface areas of several hundred square meters per gram enable the adsorption of a significant portion of the adsorbent's weight in gas.
Different molecules can have different affinities for adsorption into the pore structure or open volume of the adsorbent. This provides one mechanism for the adsorbent to discriminate between different gases. In addition to their affinity for different gases, zeolites and some types of activated carbons, called carbon molecular sieves, may utilize their characteristics to exclude or slow the diffusion of some gas molecules into their structure. This provides a mechanism for selective adsorption based on the size of the molecules and usually restricts the ability of the larger molecules to be adsorbed. Either of these mechanisms can be employed to selectively fill the micropore structure of an adsorbent with one or more species from a multi-component gas mixture. The molecular species that selectively fill the micropores or open volume of the adsorbent are typically referred to as the “heavy” components and the molecular species that do not selectively fill the micropores or open volume of the adsorbent are usually referred to as the “light” components.
An early teaching of a PSA process having a multi-bed system is found in U.S. Pat. No. 3,430,418 wherein a system having at least four beds is described. This '418 patent describes a cyclic PSA processing sequence that includes in each bed: (1) higher pressure adsorption with release of product effluent from the product end of the bed; (2) co-current depressurization to intermediate pressure with release of void space gas from the product end thereof; (3) countercurrent depressurization to a lower pressure; (4) purge; and (5) repressurization. The void space gas released during the co-current depressurization step is commonly employed for pressure equalization purposes and to provide purge gas to a bed at its lower desorption pressure. Another conventional PSA processes using three sorbent beds is disclosed in U.S. Pat. No. 3,738,087. Conventional PSA processes are typically able to recover only one of the key components (i.e., light or heavy) at high purity and are unable to make a complete separation and separate both components with a high recovery. The light component usually has a low recovery factor. Recovery of the light component usually drops even lower when the feed gas is introduced at higher pressures (i.e., pressures above 500 psig).
While there are various teachings in the art with respect to new adsorbent materials, new and improved parallel channel contactors, and improved rapid cycle PSA equipment, none of these to date present a viable solution to the problem of producing good recovery of the light component and purity when the feed gas is at very high-pressure. This is a critical issue since natural gas is often produced at high pressures (500-7000 psi) and methane acts as a light component in the adsorption process. One problem in extending PSA processes to high pressures, especially with those streams containing large amounts of CO2, is that at the end of the adsorption step there can be more moles of gas in the flow channels and mesopores then there are in the microporous sorbent. This can lead to poor recovery of desired product and also to low purity product streams.
Another problem can be that at high pressures the isotherm of an adsorbent saturates. When the isotherm saturates, the amount of material adsorbed changes slowly with increasing pressure. The slope of the isotherm, as a function of pressure, can be less than 1/100 of the slope of the isotherm at low pressures. This low slope decreases the effectiveness of the adsorbent, lowering recovery and decreasing the purity of the components recovered.
Many gas fields also contain significant levels of H2O, H2S, CO2, N2, mercaptans and/or heavy hydrocarbons that have to be removed to various degrees before the gas can be transported to market. It is preferred that as much of the acid gases such as H2S and CO2 be removed from natural gas as possible. In all natural gas separations, methane is a valuable component and acts as a light component in swing adsorption processes. Small increases in recovery of this light component can result in significant improvements in process economics and also serve to prevent unwanted resource loss. It is desirable to recover more than 80 vol %, preferably more than 90 vol % of the methane when detrimental impurities are removed. While various processes exist for removing CO2, H2S, and N2 from natural gas, there remains a need for processes and materials that will perform this recovery more efficiently, at lower costs, and at higher hydrocarbon yields, particularly at higher methane yields. For example, purification of high CO2 content natural gas using PSA technologies is challenging because of: 1) the large volume of gas that must be processed; 2) the fact that all conventional adsorbents preferentially adsorb CO2; 3) the presence of impurities such as H2S, water, and higher hydrocarbons; 4) the very high pressures that makes minimization of voidage and dead spaces of the adsorbent critical; and 5) the fugacity of CO2 that significantly affects the physics of high-pressure adsorption and transport in microporous materials.
Therefore, there remains a need in the art for the purification of high pressure gaseous streams, such as high-pressure natural gas streams containing significant amounts of CO2.